In recent years, a variety of optical recording media have been developed and optical pickup devices that carry out recording and reproducing using two alternative types of optical recording media have been known. For example, a system that carries out recording or reproducing with either a DVD (Digital Versatile Disk) or a CD (Compact Disk including CD-ROM, CD-R, CD-RW) has been used. For these two optical recording media, the DVD uses visible light having a wavelength of approximately 657 nm for improved recording densities while the CD is required to use near-infrared light having a wavelength of approximately 790 nm because there are some recording media that have no sensitivity to visible light. A single optical pickup device, known as a dual-wavelength-type pickup device, uses irradiation light of these two different wavelengths. The two optical recording media described above require different numerical apertures (NA) due to their different features. For example, the DVD is standardized to use a numerical aperture of 0.6 and the CD is standardized to use a numerical aperture in the range of 0.45-0.52. Additionally, the thicknesses of the two types of recording disks, including the thickness of a protective layer of polycarbonate (PC), are different. For example, the DVD may have a thickness of 0.6 mm and the CD may have a thickness of 1.2 mm.
Additionally, the amount of spherical aberration generated by the disk thickness varies with the thickness of the disk. Therefore, the amount of spherical aberration generated by the disk thickness is different for DVDs than for CDs. Consequently, in order to obtain precise focusing on the optical recording media regardless of the type of disk used, it is necessary to design optical systems that focus the light on the optical disks with varying amounts of convergence that take into account differences in spherical aberration for the light beams of different wavelengths that are used.
Specifically, in order to meet demands for reducing the size of the system, it is necessary to increase the degree of freedom in selecting the position of the optical system. As a result, it is considered advantageous to provide a configuration such that the light beams of different wavelengths from the light source or light sources are substantially parallel light beams when they are incident on the objective lens of the optical system that focuses the light on each optical recording medium.
For example, in Japanese Patent Application 2003-91382, the applicants of the present invention disclosed objective lenses for optical recording media that focus light beams to two different optical recording media for recording and reproducing information that strike the objective lenses as substantially parallel beams in order to result in favorable converging actions of the light beams for each of the two different optical recording media by using a optical diffractive surface as one of the objective lens surfaces. This allows much greater freedom in locating the optical system.
As the amount of data that is handled in everyday life rapidly increases, demands for increasing the amount of data optical recording media can record become urgent. In order to increase the amount of data that optical recording media can record, it is known to reduce the wavelength λ of the light used and to increase the numerical aperture (NA) of the objective lens. For producing a short wavelength, a short wavelength semiconductor laser made from gallium nitride (a GaN laser), which emits laser light with a wavelength of 405 nm, useful for providing the recording and reproducing light, is being developed and put into practical use. With the development of short wavelength semiconductor lasers, research and development of AODs (Advanced Optical Disks), also known as HD-DVDs, that provide approximately 20 GB of data storage on a single side of an optical disk by using short wavelength light is in progress. As the AOD standard, the numerical aperture and disk thickness are selected to be close to, but slightly different from, those of the above-mentioned DVDs, with the numerical aperture (NA) and disk thickness for an AOD being set at 0.65 and 0.6 mm, respectively.
Additionally, research and development of Blu-ray disk systems that use a short wavelength of disk illuminating light is underway, and the values of the numerical aperture and disk thickness for those systems are completely different from the corresponding above-mentioned DVD and CD values, with a numerical aperture (NA) of 0.85 and a disk thickness of 0.1 mm being standard. Unless otherwise indicated, hereinafter, AOD and Blu-ray disk systems will be collectively referred to as AOD and the like systems.
Therefore, the development of an optical pickup apparatus that can be used for three types of optical recording media, such as AODs and the like, DVDs and CDs is expected, and an objective lens with which such optical recording media may be used has been proposed, for example, in Japanese Laid-Open Patent Application 2003-67972. This Japanese application teaches reducing spherical aberration by taking advantage of the magnification change of the objective lens with the light beams of different wavelengths that are used, and further reducing spherical aberration by using a wavelength selective filter in recording or reproducing using different kinds of optical recording media.
However, it is difficult to satisfy the condition discussed above that each light beam of a different wavelength from a light source for illuminating one of each of three different optical recording media be incident on the objective lens as a substantially parallel beam according to the teachings of Japanese Patent Application 2003-91382 and Japanese Laid-Open Patent Application 2003-67972 discussed above. In particular, objective lenses according to Japanese Patent Application 2003-91382 and Japanese Laid-Open Patent Application 2003-67972 may be arranged so that the light beams having wavelengths in use relative to one or two types of optical recording media are incident onto the objective lens as substantially parallel beams in order to favorably record or reproduce data on the different optical recording media, but it is technically difficult to achieve a substantially parallel incident beam when three different recording media are used.
Satisfying the desired conditions for three different types of recording media depends on favorably controlling the actions of the entire objective lens for each wavelength, that is, favorably controlling refractive actions of each surface of the objective lens where refraction occurs and favorably controlling diffractive actions of each surface of the objective lens where diffraction occurs. More specifically, by accurately controlling diffractive actions, as well as refractive actions, for each wavelength at a optical diffractive surface, even, for example, when only two optical recording media satisfy the requirements for desired focusing, as discussed above, the improvements in optical imaging in general can be quite substantial.